1. Field of the Invention
The invention relates to hip prostheses and more particularly to a cotyloidal component of a hip prosthesis which is intended to be implanted without cement and which comprises a metal dome member to be fixed in a suitably prepared cotyloid cavity with a polar axis oriented substantially in the mean direction of the neck of the femur, and a cup of polymer material which is to be precisely fitted into the dome member to form a rubbing or friction lining and is to receive the spherical head of a femoral component so as to reproduce the natural joint.
2. Description of the Prior Art
The practice at one time was to fix the femoral and cotyloidal prosthesis components respectively in the femur and in the acetabulum of the iliac bone, by means of a polymerisable cement generally of the acrylic type.
That process suffered from a number of disadvantages with in particular the creation in the bones involved of lines of forces which were oriented in directions which were not natural ones, giving rise to pains, and the loss of plasticity of the sealing material in the course of time, with the result that clearance was formed between the bone and the corresponding component.
Hip prosthesis components were then produced, which were suitable for being implanted without cement, anchoring of such components in the bones which receive them being effected by strong mechanical connections.
The cotylidal components to which the present invention relates fall into three categories, in the present state of the art.
In a first category, the component comprises a metal dome or cap member of annular shape, provided with a peripheral outside screwthread which is to be engaged in a complementary thread provided in the wall of the acetabulum. The screwthreads are frequently self-tapping, with interrupted screwthread portions having cutting edges.
Positioning of the component in a prestressed condition is intended to ensure that the screwing effect is irreversible. However, osteolysis of the acetabulum sometimes occurs, resulting in the component suffering from play. In addition, the path of the lines of force from the femur to the iliac bone is somewhat different from that which occurs in the natural healthy joint, with an inclination with respect to the directions of greatest natural strength of the bone structure.
In a second category the component comprises a substantially hemispherical dome member or cap whose outside surface is provided with a covering of porous metal. That type of component, which originally was provided to improve the connection between the component and a cement, has been found to be suitable for promoting a direct connection between the metal component and the growing spongy bone which penetrates into the pores in the metal.
In fact, the connecting mechanism involving growth of the spongy bone which penetrates into the pores of the metal, commonly referred to as `rehabitation` is effective only after a period of time which is reckoned in weeks. Use is therefore made of a primary and immediate anchoring action which is achieved by means of screws which pass through passages in the dome member or cap and which are engaged into screwthreaded holes provided in the wall of the acetabulum. However the primary anchoring effect is relatively fragile, particularly when the patient suffers from bone ageing.
The first two categories of cotyloidal components suffer from the serious disadvantage of not complying with one of the basic points of mechanics insofar as deformations of the acetabulum which is subjected to tensile and compression stresses in response to the load of the hip are not followed with a sufficient degree of exactitude by the metal dome members.
A third category of cotyloidal components has been used, wherein those components are reduced to a dome member which generally speaking is of a substantially hemispherical outside shape and which is made of a polymer material having a level of elasticity close to that of the bone. More precisely, the flexibility of the polymer permits it to follow the resilient flexural movements of the bone in which it is housed. Primary anchoring is effected by means of one or more studs or pegs which are engaged into housings provided in the acetabulum.
In spite of having mechanical characteristics which are very close to those of the natural bone, that category of component is often considered to suffer from disadvantages, namely the absence of secondary anchoring by the formation of a fibrous interface between the bone and the polymer, and bleeding-out phenomena in respect of the polymer (plasticising agents) in direct contact with the tissues, which give rise to serious problems in regard to biocompatibility.
It is among the objects of the present invention to provide a cotyloidal component derived from the second category set forth above, which exhibits the following criteria:
conformity with the distribution of the natural lines of force in a healthy joint and the adjacent bones;
primary fixing of the component in the bone, in a strong and secure manner; and
precise contact with the bone which, in conjunction with biomechanical properties which are close to those of the bone, permits an excellent secondary anchoring effect by boney rehabitation.